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Sunday, October 9, 2011

I mentioned this in Ppodcast #138 and wanted to provide more info. Here are the relevant paragraphs from Sagan's book, "The Pale Blue Dot."

"Of course, there's a background level of radio noise from Earth-radioand television stations, aircraft, portable telephones, nearby and moredistant spacecraft. Also, as with all radio receivers, the longer youwait, the more likely it is that there'll be some random fluctuation inthe electronics so strong that it generates a spurious signal. So weignore anything that isn't much louder than the background. Any strong narrow-band signal that remains in a single channel we takevery seriously. As it logs in the data, META automatically tells the humanoperators to pay attention to certain signals. Over five years we madesome 60 trillion observations at various frequencies, while examining theentire accessible sky. A few dozen signals survive the culling. These aresubjected to further scrutiny, and almost all of them are rejected-forexample, because an error has been found by fault-detectionmicroprocessors that examine the signal-detection microprocessors. What's left-the strongest candidate signals after three surveys of thesky-are 11 "events." They satisfy all but one of our criteria for agenuine alien signal. But the one failed criterion is supremely important:Verifiability. We've never been able to find any of them again. We lookback at that part of the sky three minutes later and there's nothingthere. We look again the following day: nothing. Examine it a year later,or seven years later, and still there's nothing. It seems unlikely that every signal we get from alien civilizationswould turn itself off a couple of minutes after we begin listening, andnever repeat. (How would they know we're paying attention?) But, justpossibly, this is the effect of twinkling. Stars twinkle because parcelsof turbulent air are moving across the line of sight between the star andus. Sometimes these air parcels act as a lens and cause the light raysfrom a given star to converge a little, making it momentarily brighter.Similarly, astronomical radio sources may also twinkle-owing to clouds ofelectrically charged (or "ionized") gas in the great near-vacuum betweenthe stars. We observe this routinely with pulsars. Imagine a radio signal that's a little below the strength that wecould otherwise detect on Earth. Occasionally the signal will by chance betemporarily focused, amplified, and brought within the detectability rangeof our radio telescopes. The interesting thing is that the lifetimes ofsuch brightening, predicted from the physics of the interstellar gas, area few minutes-and the chance of reacquiring the signal is small. We shouldreally be pointing steadily at these coordinates in the sky, watching themfor months. Despite the fact that none of these signals repeats, there's anadditional fact about them that, every time I think about it, sends achill down my spine: 8 of the 11 best candidate signals lie in or near theplane of the Milky Way Galaxy. The five strongest are in theconstellations Cassiopeia, Monoceros, Hydra, and two in Sagittarius-in theapproximate direction of the center of the Galaxy. The Milky Way is aflat, wheel-like collection of gas and dust and stars. Its flatness is whywe see it as a band of diffuse light across the night sky. That's wherealmost all the stars in our galaxy are. If our candidate signals reallywere radio interference from Earth or some undetected glitch in thedetection electronics, we shouldn't see them preferentially when we'repointing at the Milky Way. But maybe we had an especially unlucky and misleading run ofstatistics. The probability that this correlation with the galactic planeis due merely to chance is less than half a percent. Imagine a wall-sizemap of the sky, ranging from the North Star at the top to the fainterstars toward which the Earth's south pole points at the bottom. Snakingacross this wall map are the irregular boundaries of the Milky Way. Nowsuppose that you were blindfolded and asked to throw five darts at randomat the map (with much of the southern sky, inaccessible fromMassachusetts, declared off limits). You'd have to throw the set of fivedarts more than 200 times before, by accident, you got them to fall asclosely within the precincts of the Milky Way as the five strongest METAsignals did. Without repeatable signals, though, there's no way we canconclude that we've actually found extraterrestrial intelligence. Or maybe the events we've found are caused by some new kind ofastrophysical phenomenon, something that nobody has thought of yet, bywhich not civilizations, but stars or gas clouds (or something) that dolie in the plane of the Milky Way emit strong signals in bafflingly narrowfrequency bands. Let's permit ourselves, though, a moment of extravagant speculation.Let's imagine that all our surviving events are in fact due to radiobeacons of other civilizations. Then we can estimate-from how little timewe've spent watching each piece of sky-how many such transmitters thereare in the entire Milky Way. The answer is something approaching amillion. If randomly strewn through space, the nearest of them would be afew hundred light years away, too far for them to have picked up our ownTV or radar signals yet. They would not know for another few centuriesthat a technical civilization has emerged on Earth. The Galaxy would bepulsing with life and intelligence, but-unless they're busily exploringhuge numbers of obscure star systems-wholly oblivious of what has beenhappening down here lately. A few centuries from now, after they do hearfrom us, things might get very interesting. Fortunately, we'd have manygenerations to prepare."

6 comments:

Thanks for this, it got my interest. I'm sure it's been said before but the spine chilling observation that "8 of the 11 best candidate signals lie in or near theplane of the Milky Way Galaxy" - surely that's due to the fact that there's just lots more close stars and planets to us in that plane?

Since reading of the original "Wow!" signal, I've always been fascinated with these strange and unexplained blips from space.

I've wondered, if perhaps some of them could be a sort of galactic lighthouse beacon.

An advanced civilisation takes note of all the candidate stars and then "pings" each star briefly.

Given there are many possible candidate stars - it could take quite a while for it to repeat.

I e-mailed Seth Shostak asking if SETI had ever spent a say a period of 6 months entirely focused on one of these points, he was nice enough to reply and the simple answer is they can't - it costs too much and there's so much other science vying to be done on a limited number of instruments.

They have gone back, they have searched for repetition, but only for short periods of time.

To me it's a shame, but I understand the predicament.

If I ever win the lotto, I'll investigate hiring a bunch of homebrewers to build a dish station to monitor the source of each blip for a solid year or two :P

I was thinking along the same lines as Dave in the comments. I know there are a group of amateur radio telescope people, many of which are hams. And probably would not have access to as high-powered of facilities as the pros. BUT, they do have the advantage of time. Why not have these folks focus steadily on some of these "candidate" signals, looking for repeats, over the course of months or even a year? And maybe this is already happening on some level?

This would be a topic I myself would like to pursue. I mean, I've sat and listened to static on the 6 m band for months on end. This wouldn't be a lot different but the payout would be enormous if that static broke with signals from other lifeforms. And repeatability/believability would be improved if others in the ham network could verify one ham's measurements before relaying back to the pros.

There's a group called "The SETI League" (http://www.setileague.org/) who make use of 3M-5M dishes in their back yards - they seem to be focused on doing an all sky survey like the SETI Institute, rather than the prolonged focus on one point that we're talking about.

I would be very interested in knowing the strengths of the previously detected signals - and whether a small dish with modern DSP would be able to detect a recurrence.

It could be totally automated and fairly simple, a small computer running a rotator, perhaps only monitoring the previously detected frequencies to cut down on the amount of data that would need to be chewed through.

I'd really love to set up a SETI-type station like shown in their block diagram on their website, and possibly do this exact thing we're discussing. The technical challenge of just getting it running would be fun. I'm not sure on the logistics of setting up some of it at my current QTH but I think it could be possible.

I've also got access to the club shack at the university here, and this is something the club could certainly do, if there's interest. We also happen to be just next door to the NRAO headquarters! lol. So there is some pretty high-IQ resources in this area very close by.

That SETI league website has a page here: http://www.setileague.org/editor/possible.htm that makes it sound a lot less possible. But then, why not have a large number of people with the equipment: a dish, radio equipment, computers, etc pointed at one location and do something like this: http://en.wikipedia.org/wiki/Aperture_synthesis

With the computers you could automate some data sharing with P2P techniques and with a lot of dishes you could probably get the combined benefit of one large one.

(I've been waiting on a P2P sort of cooperation between web based SDRs for awhile now. Consider the benefit if you could check your signal on many receivers in your area, or around the world. I guess there's things like WSPR and reverse beacon networks that kinda do that already though.)

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